The present invention relates to an injection device used for molding a resin.
A known injection device used for molding a resin comprises a cylinder having an injection nozzle at an end thereof, a plunger sliding in the axial direction in a position opposite to the injection nozzle within the cylinder, and a driving device for advancing and retracting the plunger. The cylinder has a port for supplying a molding material between the sliding position of the plunger and the injection nozzle, and the molding material supplied into the cylinder is compressed by advancement of the plunger, so that the molding material is injected into a cavity of molds connected to the injection nozzle. In such an injection device, a screw for melting the molding material is provided outside the cylinder in the case of a pre-plasticizing type, and is provided integrally with a leading end of the plunger within the cylinder in the case of an in-line type. The plunger of such an injection device of the pre-plasticizing type may be provided with a larger diameter portion in contact with an inner circumferential wall surface of the cylinder for isolating inner and outer regions of the cylinder from each other and a smaller diameter portion in a leading end thereof.
The above injection device of a conventional injection molding machine has presented such issues as described below.
When the molds are opened for removing a product, a molding material is forced out of a leading end of the injection nozzle, causing a so-called drooling condition. Therefore, if the pressure (back pressure) of the molding material in the side of a plunger head is too high, while weighing comes to be unstable, or if the back pressure is too low, a suck-back operation has been conducted for moving the plunger slightly further backward from a position of the plunger after it has been pushed back by the back pressure to an extent sufficient to prevent the drooling.
In order to prevent the molding material from leaking out of the cylinder through a clearance (gap) between the cylinder and the plunger at the time the molding material is supplied into the cylinder, the molding material is injected into the mold cavity and the like, the length of a portion of the plunger in contact with the cylinder has been made longer. In addition, the clearance between the plunger and the cylinder has been reduced, that is, narrowed to such a minimum extent to allow the plunger to slide.
In the cylinder, a check valve (ring valve) has been employed between a fluid holding part with the injection nozzle and the supply port for preventing a molten liquid of molding material held in the fluid holding part from flowing back to the supply port. The check valve is brought into tight contact with a valve seat of the plunger by the back pressure of the molten liquid of molding material, when the backward movement of the plunger is discontinued after weighing, so that back flow of the molten liquid to the supply port is prevented.
On the other hand, in an attempt to prevent back flow of the molding material from the mold cavity to an inner region of the cylinder until completion of a pressure-holding and compression process after injection of the molding material to the mold cavity, a pressure control has been conducted by applying a force to the plunger in the advancing direction of the plunger until completion of the pressure-holding and compression process after injection of the molding material to the mold cavity. Therefore, a force of the molding material flowing back from the mold cavity into the cylinder is counterbalanced with the force applied to the plunger.
Such an injection device according to the prior art, however, has the below-mentioned issues.
During the suck-back operation, because the inner region of the cylinder is reduced in pressure, the air is brought into the cylinder from outside the cylinder through a trailing end of the plunger, and the air is entrapped in a product as a void (bubble), causing a defect in the product. Although P2 greater than P0 at the time of weighing of the molding material, P2 less than P0 after completion of the weighing where a pressure of molding material from the plunger head to a guide is P1, a pressure of the molding material (back pressure) in the side of a leading end of the plunger head is P2, and a pressure outside the cylinder is P0 (atmospheric pressure). Entrainment of the air from outside the cylinder is caused at P1 less than P0 due to such reason as insufficient closing of the check-valve.
Even if the clearance between the plunger and the cylinder is narrowed, the molding material is adhered to the wall surfaces of the plunger and the cylinder in the sliding zone of the plunger, and it is unavoidable that the molding material enters between the plunger and the cylinder. Moreover, once the molding material enters between the plunger and the cylinder, it does not readily flow between them, because the clearance is very narrow, and retention of the molding material in the clearance is caused. The molding material retained therein is reduced in quality and carbonized due to a frictional heat caused by sliding movement of the plunger. Then, the carbide is entrained in the molding material in the cylinder, and contaminates a molded product, causing a defect in the product.
Shown in FIG. 8A is an injection device of pre-plasticizing type comprising a plunger 2 with a larger diameter portion 21 in contact with an inner circumferential wall surface of a cylinder 1 for isolating inner and outer regions of the cylinder from each other, and a smaller diameter portion 22 in a leading end thereof. A molten liquid of molding material fed from a port 12 for supplying the molding material in the cylinder 1 is guided toward an injection nozzle (lower side of the figure, not shown) by a stepped surface 23 in a boundary between the larger diameter portion 21 and the smaller diameter portion 22. However, a part of the molten liquid of molding material supplied from the molding material supply port 12 is divided into two streams flowing in the right and left directions about the smaller diameter portion 22 of the plunger 2 as shown in FIG. 8B, and these streams collide against each other in a position backside in relation to the supply port 12. This collision causes a stagnating area 9 at that position. Then, the molding material in the stagnating area 9 is reduced in quality and carbonized by heat, and the carbide is removed from the wall surface of the smaller diameter portion 22, and contaminates a product, causing a defect in the product.
The check valve is not brought into sufficiently tight contact with the valve seat of the plunger, that is, it is not completely closed, even when the backward movement of the plunger is discontinued after completion of weighing, and is completely closed only after injection is completed. Therefore, a significant and unstable quantity of molten liquid of the molding material flows back before the valve is completely closed. As a result, a present quantity of the molding material to be supplied and an actual quantity of molding material supplied are unequal to each other, because the actual quantity supplied has been varied. Thus, it has been difficult to assure a fixed quantity of molding material to be supplied into the mold cavity.
In the pressure control described above, the position of the plunger in the cylinder should be stable. However, as the pressure of the molding material in the mold cavity during the pressure-holding and compression process and the force applied to the plunger in the advancing direction of the plunger are balanced with each other, the balance between them is actually varied in a subtle manner. Therefore, the position of the plunger is varied, so that it has been difficult to consistently maintain the forces in perfectly balanced condition in order to provide a constant holding pressure. Accordingly, the molding material is forced in and out of the molding cavity during the pressure-holding and compression process, and it has been difficult to assure that a fixed quantity of molding material is supplied into the cavity.
Accordingly, an object of the present invention is to provide an injection device capable of eliminating defects in a molding due to entrainment of carbides and voids and difficulty in assuring a fixed quantity of molding material to be supplied into a mold cavity.
In accomplishing these and other aspects, according to one aspect of the present invention, there is provided an injection device comprising a cylinder, a plunger, and a driving device. The cylinder has an injection nozzle at an end thereof. The plunger is capable of sliding in an axial direction thereof in a position opposite to the injection nozzle within the cylinder, and the driving device advances and retracts the plunger.
The cylinder has a port for supplying a molding material between a sliding position of the plunger and the injection nozzle, and the molding material supplied into the cylinder is compressed by advancement of the plunger. Thus, the molding material is injected into a cavity of a mold connected to the injection nozzle. In addition, the plunger has a groove circumferentially extending along an outer circumferential wall surface thereof. The injection device may be either of a pre-plasticizing type or in-line type.
A pressure in an inner region X of the cylinder is Px, a pressure in an outer region Z of the cylinder is Pz, and a pressure in an area Y of the plunger is Py. The air in the outer region of the cylinder cannot be drawn into the inner region of the cylinder if the inner region of the cylinder is at a positive pressure (+Px), because a pressure gradient shown by a solid line in FIG. 6, that is, a gradient of (a positive pressure +Py) in the area Y of plunger is present between the inner and outer regions of the cylinder. However, the air in the outer region of the cylinder is drawn into the inner region of the cylinder, if no groove is provided in the outer circumferential wall surface of the plunger when the inner region of the cylinder is at a negative pressure (xe2x88x92Px), because a pressure gradient shown by a dotted line in FIG. 6, that is, a gradient of (a negative pressure xe2x88x92Py) in the area of plunger is present between the inner and outer regions of the cylinder. In contrast, if a groove is provided in the outer circumferential wall surface of the plunger, as shown in an alternate long and short dash line in FIG. 6, a positive pressure (+Py) can be temporarily maintained in an area of the groove, and the air is prevented from being drawn into the inner region of the cylinder.
Although one such groove may be provided, it is preferable to provide a plurality of such grooves for the purpose of increasing an effect of preventing the air from being drawn from outside the cylinder. Although the groove may have any sectional shape, in order to prevent retention of a molding material in a bottom part of the groove, it is preferably formed with round corners in the bottom part of the groove.
The injection device of a second aspect of the present invention is characterized in that it is a pre-plasticizing type. The plunger has a smaller diameter portion in a leading part and a larger diameter portion in a trailing part thereof. An outer circumferential wall surface of the larger diameter portion of the plunger is in contact with an inner circumferential wall surface of the cylinder for isolating inner and outer regions of the cylinder from each other. Thus, a molding material is guided toward the injection nozzle by a stepped surface between the smaller diameter portion and the larger diameter portion. If one groove is provided in the plunger, a clearance between the outer circumferential wall surface of the larger diameter portion of the plunger and the inner circumferential wall surface of the cylinder is defined by a below-described equation 1 where xcex7 is a viscosity of the molding material, P is a back pressure (kgf/cm2) during weighing, Q is a quantity (cm3/s) of the molding material to be supplied, R is an inner radius of the cylinder, r1 is an outer radius of the smaller diameter portion, r2 is an outer radius of a part of the larger diameter portion before the groove, r3 is an outer radius of a bottom surface of the groove, r4 is an outer radius of a part of the larger diameter portion after the groove, l1 is an axial length of the smaller diameter portion, l2 is an axial length of the part of the larger diameter portion before the groove, l3 is an axial length (width) of the groove, l4 is an axial length of the part of the larger diameter portion after the groove, and k is the ratio of a flow rate in the upward direction of the plunger from a port for supplying the molding material into the cylinder to a total quantity of the molding material supplied.
xe2x80x83xcfx80P/2xcex7Q+{(1xe2x88x92k)l1/[(R4xe2x88x92r14)xe2x88x92(R2xe2x88x92r12)/(l1Rxe2x88x92l1r1)]}
=kl2/[(R4xe2x88x92r24)xe2x88x92(R2xe2x88x92r22)/(l2Rxe2x88x92l2r2)]
+kl3/[(R4xe2x88x92r34)xe2x88x92(R2xe2x88x92r32)/(l3Rxe2x88x92l3r3)]
+kl4/[(R4xe2x88x92r44)xe2x88x92(R2xe2x88x92r42)/(l4Rxe2x88x92l4r4)]xe2x80x83xe2x80x83[Equation 1]
By providing the smaller diameter portion and the larger diameter portion in the plunger, and guiding the molding material toward the injection nozzle by the stepped surface in the boundary between them, back flow of a molten liquid of the molding material is reduced. For smoothly effecting the guiding operation, it is preferable that the molding material supply port is partly covered by the larger diameter portion when the molding material is supplied from the molding material supply port.
By defining the clearance between the outer circumferential wall surface of the larger diameter portion of the plunger and the inner circumferential wall surface of the cylinder as set forth above, the molten liquid of molding material can flow more easily in the clearance in comparison with the prior art, and the wall surfaces are washed by the molding material, so that stagnation of the molding material is reduced. Degradation and carbonization of the molding material can thereby be prevented.
In this regard, although the length of the larger diameter portion of the plunger should be shorter and the clearance should be increased in order to prevent stagnation of the molding material, the length of the larger diameter portion of the plunger should be longer and the clearance should be reduced in order to prevent entrainment of air from the outer region of the cylinder to the inner region of the cylinder. Thus, the parameters are in close relation with each other, and the inventors, as a result of studies, have obtained the above equation 1 for determining an optimum value of the clearance.
The equation 1 could be derived from equations shown below. That is, the following equation 2 is given where pressure losses in parts A, B, C, and D in a sliding zone of the plunger 2 within the cylinder 1 are PA, PB, PC, and PD, as shown in FIG. 4.
dPA/dx=(2xcex7Q1/xcfx80){1/[(R4xe2x88x92r14)xe2x88x92(R2xe2x88x92r12)/(l1Rxe2x88x92l1r1)]}
dPB/dx=(2xcex7Q2/xcfx80){1/[(R4xe2x88x92r24)xe2x88x92(R2xe2x88x92r22)/(l2Rxe2x88x92l2r2)]}
dPC/dx=(2xcex7Q2/xcfx80){1/[(R4xe2x88x92r34)xe2x88x92(R2xe2x88x92r32)/(l3Rxe2x88x92l3r3)]}
dPD/dx=(2xcex7Q2/xcfx80){1/[(R4xe2x88x92r44)xe2x88x92(R2xe2x88x92r42)/(l4Rxe2x88x92l4r4)]}xe2x80x83xe2x80x83[Equation 2]
Therefore, the equation 3 is established.
PA=(2xcex7Q1/xcfx80){l1/[(R4xe2x88x92r14)xe2x88x92(R2xe2x88x92r12)/(l1Rxe2x88x92l1r1)]}
PB=(2xcex7Q2/xcfx80){l2/[(R4xe2x88x92r24)xe2x88x92(R2xe2x88x92r22)/(l2Rxe2x88x92l2r2)]}
PC=(2xcex7Q2/xcfx80){l3/[(R4xe2x88x92r34)xe2x88x92(R2xe2x88x92r32)/(l3Rxe2x88x92l3r3)]}
PD=(2xcex7Q2/xcfx80){l4/[(R4xe2x88x92r44)xe2x88x92(R2xe2x88x92r42)/(l4Rxe2x88x92l4r4)]}xe2x80x83xe2x80x83[Equation 3]
Here, since Q=Q1+Q2, P+PA=PB+PC+PD, and Q1=(1xe2x88x92k)Q, if Q2=kQ, a below-described equation 4 is given.
P+(2xcex7(1xe2x88x92k)Ql1/xcfx80{(R4xe2x88x92r14)xe2x88x92(R2xe2x88x92r12)/(l1Rxe2x88x92l1r1)}
=(2xcex7kQl2)/xcfx80{(R4xe2x88x92r24)xe2x88x92(R2xe2x88x92r22)/(l2Rxe2x88x92l2r2)}
+(2xcex7kQl3)/xcfx80{(R4xe2x88x92r34)xe2x88x92(R2xe2x88x92r32)/(l3Rxe2x88x92l3r3)}
+(2xcex7kQl4)/xcfx80{(R4xe2x88x92r44)xe2x88x92(R2xe2x88x92r42)/(l4Rxe2x88x92l4r4)}xe2x80x83xe2x80x83[Equation 4]
In such a manner, the first mentioned equation 1 is derived.
An injection device according to an aspect of the invention is further characterized in that it is a pre-plasticizing type. A guide element is provided in the smaller diameter portion of the plunger in a backside position in relation to the supply port for guiding two streams of a molten liquid of molding material. The streams flow circumferentially about the smaller diameter portion to reach the backside position, toward the injection nozzle. In addition, the guide element preferably has a pair of inclined guide surfaces which have approximately the same inclined angle with respect to the axial direction of the plunger. Alternatively, the guide element is preferably formed in a generally triangular shape at a protruding end of its apex, although it is not specifically limited to such form. In such case, two opposite oblique lines of the triangle can be curved for smoother flow of the molten liquid.
The protrusion of the guide element is only required to be convex in the advancing direction of plunger so that it matches the flowing direction of the molding material in order to prevent stagnation of the molding material thereabout. It may, therefore, be in the form of a partition plate, since the stagnating area of molding material is generally triangular, and a relation of xy=c (where x is a base of the shape similar to a triangle of molding material stagnating area (a part of the plunger bottom surface), y is a height of the shape similar to the triangle, and c depends on a viscosity of the molding material and the like) is given, it is preferably formed in the triangular shape, and most preferably such shape that meets the equation. It is further preferable that the convex portion is progressively reduced in thickness toward the apex thereof, and has no thickness at the apex.
The backside position of the smaller diameter portion is preferably located in such a position that occupies an upper half of the molding material supply port in view of preventing back flow and stagnation and facilitating flow of the molding material in the head direction of the plunger, although it is not specifically limited thereto.
In order to assure a fixed quantity of molding material will be supplied into a mold cavity, an injection device of an aspect of the invention is still further characterized in that a check valve is employed between a fluid holding part with the injection nozzle and the supply port for preventing back flow of a molten liquid of molding material held in the fluid holding part to the supply port. The check valve is continuously forced against a valve seat provided in the plunger by means of a spring, which is located in a connecting shaft extending frontward from the plunger. Further, the injection device may be either of a preplasticizing type or an in-line type.
In such case, although the material of the spring is not specifically limited, for example, the spring may be a metal spring or the like. In addition, a coil spring or the like as an example can be employed without limitation to any specific shape. As for the number of springs, at least one spring is required.
Finally, an injection device of an aspect of the invention is characterized in that the driving device is further provided with a control means having a position detector for detecting a position of the plunger within the cylinder in order to control the operation of the driving device according to a detection signal thereof for fixing the position of the plunger at a position at the end of the injection process. Further, the injection device may be either of a pre-plasticizing type or an in-line type.